Revit works with real measurement. It doesn’t matter if you set your units to metric or imperial, you can still work with another unit.
No, you don’t need to change the project units. You can add the unit as the suffix regardless of the project units setting. For example, you can type 6m instead of 6000 when you use the metric unit.
Revit also allows you to use a formula when providing input. Type = then the formula that you want to use. For example, = 10m/3.
Watch how the unit suffix and the formula works in Revit.